1. Field of the Invention
The present invention relates generally to a digital automatic fine tuning method and apparatus, which is capable of detecting a difference between a nominal frequency and an intermediate frequency generated in a tuner using a counter.
2. Description of the Prior Art
In general, apparatuses for processing broadcasting signals, such as televisions, Video Cassette Recorder (VCRs) or set-top boxes, are each equipped with a tuner for receiving high frequency broadcasting signals of a selected channel.
FIG. 1 shows a schematic block diagram of the tuner, where RF sources are high frequency broadcasting signals of a plurality of channels, including modulated image, color, and voice carrier. In this tuner, a Radio Frequency (RF) amplifying unit 11 receives the RF sources, and selects and amplifies a RF signal of the RF sources corresponding to a channel selected by a user. A mixing unit 12 combines the selected RF signal with a local oscillation signal LO having a frequency corresponding to the selected channel to produce an Intermediate Frequency (IF) signal IF. The IF signal IF is an output signal of the tuner, and is applied to a next image/voice processing unit 14 to be separated into an image signal and a voice signal.
In that case, when the user selects the channel through a user manipulation unit 19 so that the RF amplifying unit 11 tunes to and amplifies the RF signal of the channel selected by the user and the mixing unit 12 generates the local oscillation frequency (fLO=fRF−fIF) required to convert a frequency of the channel selected by the user into a predetermined IF fIF, a microprocessor 18 informs a tuning voltage generating unit 16 of the selected channel and the tuning voltage generating unit 16 applies a tuning voltage VT corresponding to the selected channel to the RF amplifying unit 11 and a local oscillation unit 15, respectively, so that a tuning frequency of the RF amplifying unit 11 and an oscillation frequency of the local oscillation unit 15 can be varied.
In general, a carrier frequency of a broadcasting signal may be varied when the broadcasting signal is broadcasted from a broadcasting station. In addition, in the tuner, the oscillation frequency of the local oscillation unit 15 may be varied. In this case, the IF produced in the mixing unit 12 may be changed, which results in the distortion of an image.
In the prior art, in order to place the IF produced in the mixing unit 12 on the nominal frequency (refer to a predetermined IF; for example, in the case of a NTSC broadcasting, it is set to 45.75 MHz for an image signal, and 41.25 MHz for a voice signal) by reducing a difference between these frequencies, the tuner further includes an automatic fine tuning unit 17 that detects an IF of a signal applied to the image/voice processing unit 14, calculates a difference between the detected IF and a predetermined nominal frequency, and transmits the calculated difference to the microprocessor 18 that calculates an automatic fine tuning value based on the difference received from the automatic fine tuning unit 17 and transmits the calculated automatic fine tuning value to the tuning voltage generating unit 16. Then, the tuning voltage generating unit 16 corrects the tuning voltage by the automatic fine tuning value, so that the IF is coincident with the predetermined nominal frequency.
The automatic fine tuning unit 17 is implemented as an analog circuit that uses an output frequency of a Voltage Controlled Oscillator (VCO) having an Inductance/Capacitance (LC) tank circuit as its input and tunes a magnitude of inductance L at the outside of an Integrated Circuit (IC). However, since it is impossible to integrate the LC tank circuit in the IC, there was proposed a digital automatic fine tuning apparatus that uses a VCO, in which resistors and capacitors are integrated in an IC, according to the trend of downsizing and multifunction and does not require any external tuning.
However, although this digital automatic fine tuning apparatus does not have an analog circuit such as an LC tank circuit, a problem arises in that a logic circuit for performing the automatic fine tuning function as described above becomes complicated. Accordingly, there is a need for a new digital automatic fine tuning apparatus with a simplified construction.